2 oz butter
9 oz cream cheese
3 oz whole milk
2 oz unbleached flour
1 oz cornstarch
1/4 t salt
1 T lemon juice
5 oz sugar
6 eggs, separated
1/4 t cream of tartar
2 T unsweetened cocoa powder
Lightly grease and line the bottom and sides of a round 12 inch springform pan with parchment paper. Wrap two layers of foil around pan to prevent water seeping in. Use a baking dish that will accommodate the springform pan and fill empty baking dish 1/4 full with water and place the baking dish with water (do not add springform) on center rack of oven. Set oven to 325.
Melt cream cheese, butter and milk over a double boiler over medium heat. Whisk constantly until melted and no lumps remain. Cool mixture over an ice bath. Fold in flour, cornstarch, 6 egg yolks, and lemon juice, mix well and set aside.
Whisk 6 egg whites with cream of tartar until foamy. Add in sugar and whisk on high speed until soft peaks form. Add one third of the egg white mixture to the cheese mixture to loosen. Fold in remaining egg white mixture in two batches and mix well. Divide batter evenly in two and sift cocoa into one bowl.
Take prepared springform pan and place 3 tablespoons of vanilla batter in a circle in the center of the tin. Then take 3 tablespoons of the chocolate batter and place in the center. Continue adding 3 tablespoon circles of batter until both of the batters are used. Circles will spread automatically.
Carefully transfer cheesecake into the water bath in oven making sure not to tilt cheesecake. Bake cheesecake in the water bath 55 to 65 minutes or until a skewer inserted in the centre comes out clean. Cool on cooling rack and refrigerate until thoroughly chilled. Serves 12.